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MINING AND METALLURGY INSTITUTE BOR ISSN: 2334-8836 (tampano izdanje)UDK: 622 ISSN: 2406-1395 (Online)
UDK: 542:66.087:546.76(045)=111 DOI:10.5937/MMEB1502149J
Bore Jegdi*, Maja Stevanovi, Aleksandar Jegdi*
CHEMICAL AND ELECTROCHEMICAL DISSOLUTION OF
CHROMIUM AT ROOM AND ELEVATED TEMPERATURES**
Abstract
The influence of temperature on electrochemical and chemical dissolution of chromium was studiedin the acidic sulphuric solutions. The hydrogen evolution reaction, as well as the anodic dissolution and
chemical dissolution of chromium from activated surfaces in sulphuric acid solutions pH 1 follow theArrhenius law, with the apparent activation energies of 35 kJ mol-1, 58 kJ mol-1and 62 kJ mol-1, respec-tively. The higher activation energy of the chemical dissolution of chromium leads to the signifycantlynoticeable chemical corrosion on elevated temperatures in comparison with the electrochemical corro-
sion.Keywords: chromium chemical corrosion, electrochemical dissolution, activation energy
*Institute for Chemistry, Technology and Metallurgy, IHTM, University of Belgrade, Njegoeva 12,Belgrade, Serbia, E-mail: [email protected]
**The work was funded by the Ministry of Education and Science and Technological Development ofthe Republic of Serbia, Project No. 34028.
INTRODUCTION
Simultaneously with the electrochemical
dissolution of chromium occurs its chemical
dissolution, which does not depend on po-
tential and that, in certain circumstances, is
dominant process of dissolution, especiallyat elevated temperatures [1-9]. Chemical
dissolution is the cause of occurrence the
hydrogen evolution, which is not subjected
to the laws of electrochemical kinetics.
Determination the activation energy of
chemical chromium dissolution, electroche-
mical chromium dissolution, cathodic hy-
drogen evolution and temperature depend-
ence on electrochemical corrosion rate was
determined and analyzed in this paper. Also,
texture of chromium was determined usingthe OIM method (orientation imaging mi-
croscopy). Determination of chromium con-centration in solution was carried out by the
atomic absorption spectroscopy (AAS). The
anodic and cathodic polarization curves
were recorded, and also, electrochemical
corrosion rates were determined using the
various electrochemical methods.
EXPERIMENTAL PART
Before testing, the surface of chromiumsamples was mechanically polished gradu-ally with abrasive paper to the grade 1000.The solution for testing was 0.1 M Na2SO4+H2SO4, pH 1, which was purified condu-cting through testing solution of pure nitro-gen.
A three-part glass electrochemical cellwith water jackets for thermostationing, withan auxiliary Pt electrode and a saturatedcalomel electrode (SCE) as the referenceelectrode were used for electrochemical tests.
The chromium electrode was cathodi-cally activated at -0.900 V for 120 s to the
aim of removal thesurface oxide layer before
each test. Oxide layer is spontaneously cre
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ates on the chromium surface in contact with
air. Electrochemical measurements were
made using the potentiostat-galvanostat
PAR 273.
RESULTS AND DISCUSION
Figure 1 shows the inverse polar image
of chromium, obtained by the OIM method.
The grains with the orientation (111) are
marked with blue colour and it can be seen
that a large number of grains on the chro-
mium surface has exactly this orientation.
The size of chromium crystal grains was
also determined, which have been the small
dimensions, with a high degree of in orien-
tations. In fact, these are separate grains
with their own orientation.
Figure 1Texture (111) of the chromium electrode is obtained by the OIM method.
The approximate grain size can be estimated on the basis of the scale given in Figure
The temperature influence on chemical
and electrochemical chromium dissolutionin temperature the range from 291 to 353 K
in aqueous sulphuric acid, pH 1 was carried
out.
Cathodic polarization curves, recorded at
different temperatures at 291 to 343 K in
sulphuric acid pH 1, are shown in Figure 2.Figure 2 shows that with an increase in tem-
perature, the rate of hydrogen evolution in
creases, while the Tafel slope variation with
the temperature was at theoretically ex-pected levels. An inset in Figure 2 shows the
dependence of current density logarithm as a
function of reciprocal temperature for de-
termination the apparent activation energy
for hydrogen evolution at -0.860 V. The
apparent activation energy 35 kJ mol-1
isfor cathodic reaction of hydrogen evolu-
tion.
Figure 2Chatodic polarization curves for chromium electrode in aqueous solution of0.1 M Na2SO4+ H2SO4, pH 1 at various temperatures
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The anodic polarization curves of chro-
mium in the solution of sodium sulphate and
sulphuric acid pH 1, recorded at different
temperatures, were shown Figure 3. The
shape of polarization curves is similar, ex-
cept that the curves at higher temperatures
are shifted towards higher current densi-
ties, as it might be expected. The Arrhe-
nius dependence at -0.750 V is shown
in inset in Figure 3. The apparent activa-
tion energyEa,an= 58 kJ mol-1
obtained for
the reaction of the active anodic dissolu-
tion. This indicates that the anodic reac-
tion is more dependent on temperature
than the cathodic reaction. The corrosion
potential change with temperature in a
direction of more negative potentials also
explains this.
Figure 3Anodic polarization curves of chromium electrode in sulphuricacid solution pH 1, at various temperatures
The electrochemical current density
jcor,el is determined by extrapolation the
Tafel slope on corrosion potential and by
the Stern-Gery linear polarization method,
while the total chromium dissolution rate
jtot, was determined analyzing the solutionwith atomic absorption spectrophotometry,
and they are shown in Figure 4. It can be
seen from Figure 4 that the overall rate of
dissolution or equivalent current density
with temperature is higher than the electro
chemical corrosion current density. A sig-
nificant part of electrochemical corrosion is
the chemical corrosion of chromium at all
temperatures. That difference increases
with temperature as it can be seen in Figure
4. At 353 K, the total dissolution rate isabout four times higher than the electro-
chemical corrosion rate. It means that the
chemical dissolution rate at this tempera-
ture is for approximately three times higher
than the electrochemical dissolution rate.
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Figure 4Total current density obtained analytically () and corrosion current density obtainedelectrochemically at different temperatures () for the chromium electrode
As it is shown in Figure 4, the overall
corrosion rate jtotat all temperatures that are
used in this experiment are higher than the
electrochemical corrosion ratesjcor,el. This is
the result of chemical dissolution of chro-
mium, which takes place directly by reactionof water molecules and atoms of chromium
from the electrode surface and not depend-
ent on potentials. Differences between the
total chromium dissolution rate determined
analyticallyjtot,analand electrochemical disso-
lution rate jcor,el vs. the reciprocal tempera-ture is plotted on the inset in Figure 4. The
apparent activation energy was determined
from the slope of this linear dependence and
it is 62 kJ mol-1
. The high value of activation
energy indicate that the influence of tem-
perature on reaction of chemical dissolution
is large and that contribution of chemical
corrosion to the overall corrosion rate is
higher at the elevated than the lower tem-
peratures. This should be kept in mind if
using data for corrosion rate, obtained at
room temperature, for predicting the corro-
sion behaviour at elevated temperatures. The
similar differences in terms of temperatureinfluence likely existing in case of corrosion
of other metals that corrode electrochemi-
cally and chemically (i.e., Fe, Ni, Mn and
other metals).
After immersion into testing solution,
the chromium electrode is in a passivestate with the characteristic corrosion po-
tentialEcor,1. During the cathodic polariza-
tion at -0.900 V, for several tens of sec-
onds, the passive films are dissolved,
whereby the electrodes activated and the
corrosion potentialEcor,2is then formed.
The chatodic curves recorded on pas-
sive chromium in form of the Tafel dia
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grams for different temperatures are pre-
sented in Figure 5. It can be noted that
cathodic Tafel slope is approximately
-0.060 V dec-1
and that the current density
increases with temperature. The inset in Fi-
gure 5 shows the Arrhenius dependence,
obtained at -0.520 V. The apparent acti-
vation energy was determined, that is
29 kJ mol-1
. Slightly lower value of activa-
tion energy is than obtained in the process of
hydrogen evaluation on the activated chro-
mium surface (35 kJ mol-1
).
Figure 5The chatodic Tafel plots for passivated chromium electrode at different temperature
Accordingly, the corrosion of active me-tallic chromium in an aqueous solution of
sulphuric acid pH 1 consists of two simulta-
neous processes corrosion, the electroche-
mical corrosion and chemical dissolution.
With increase of temperate, significantlyincreases the anodic dissolution, chemical
dissolution, and cathodic hydrogen evolution
on the bare chromium surface, as well as on
the surface coated with chromium oxide in
0.1 M Na2SO4+ H2SO4 solution, pH 1.
Reaction of the hydrogen evolution, an-
odic dissolution and chemical dissolution onthe activated chromium surface in sulp-huric
acid solution, pH 1 in temperature range of
293 to 353 K followed the Arrhe-nius de-
pendence with an apparent activation energy
of 35 kJ mol-1
.58 kJ mol-1 and 62 kJ mol
-1,
respectively. The apparent activation energy
of the hydrogen evolution reaction on chro-
mium oxide covered surface has value of29 kJ mol
-1.
CONCLUSION
Corrosion of the active metallic chro-
mium in aqueous sulphuric acid pH 1 simul-
taneously flowed by electrochemical and
chemical mechanism. The rate of che-mical
corrosion is obtained as difference of the
total and electrochemical corrosion rate.
Reaction of the hydrogen evolution, an-
odic dissolution and chemical dissolution ofCr from chromium-activated surface in sul-
phuric acid solution, pH 1 at temperature
from 291 to 353 K followed the Arrhenius
law from the apparent activation energy of
35 kJ mol-1, 58 kJ mol
-1 and 62 kJ mol
-1,
respectively. A high activation energy of
chemical dissolution of chromium, at ele
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vated temperatures causes to significantly
higher chemical corrosion, compared to
electrochemical corrosion. The apparent
activation energy of hydrogen evolution on
the chromium-covered oxide layer has a
value of 29 kJ mol-1.
As the result of simultaneous occurrence
of reaction of hydrogen evolution on the
oxide-covered chromium surface and rea-
ction of anodic dissolution of chromium
through the passive film, the stable corrosion
potentialEcor.1was formed. The other corro-
sion potential was formed as the results of
cathodic hydrogen evolution and anodicdissolution on the bare chromium surfaces,
when the chromium surface is depasivated
(by cathodic activation, or mechanical ac-
tion, etc.). In this case, the stable corrosion
potentialEcor.2establishes.
REFERENCES
[1] Wilde B. E., Hodge F. G., The Catho-
dic Discharge of Hydrogen on Active
and Passive Chromium Surfaces in
Dilute Sulphuric Acid Solutions, Ele-
ctrochim. Acta 14 (7) (1969) pp. 619-627;
[2] Kolotyrkin Ya. M., Florianovich G.
M., Anomalnoe rastvorenie metallov.
Eksperimentalnie fakti i ih teore-
ticheskoe tolkovanie, Zashch. Metal.
20 (1) (1984) pp. 14-24;
[3] Drai D.M., Popi J.P., AnomalousDissolution of Metals and Chemical
Corrosion (Review), J. Serb. Chem.
Soc. 70 (3) (2005) pp. 489-513;
[4] Drai D.M., Popi J.P., Dissolution ofChromium in Sulfuric Acid, J. Serb.
Chem. Soc. 67 (11) (2002) pp. 777-783;[5] Popi J. P., Drai D. M., Electro-
chemistry of Active Chromium. Part III.
Effect of Temperature, J. Serb. Chem.
Soc. 68 (11) (2003) pp. 871-883;
[6] Drai D. M., Popi J. P., Electro-chemistry of Active Chromium: Part
1Anomalous Corrosion and Pro-ducts of Chromium Dissolution in
Deaerated Sulfuric Acid, Corrosion 60
(3) (2004) pp. 297-304.
[7] Popi J. P., Drai D. M., Electro-chemistry of Active Chromium: Part
II. Three Hydrogen Evolution Reac-
tions on Chromium in Sulfuric Acid,Electrochim. Acta 49 (27) (2004) pp.
4877-4891;[8] Drai D. M., Popi J. P., Jegdi B.,
Vasiljevi-Radovi D., Electroche-mistry of Active Chromium. Part IV.
Dissolution of Chromium in Deaerated
Sulfuric Acid, J. Serb. Chem. Soc. 69
(12) (2004) pp. 1099-1110;
[9] Sukhotin A. M., Khoreva N. K., Passiv-
nost khroma. Osobenosti katodnogo
aktivirovaniya khroma, Electrokhimiya
18 (1) (1982) pp. 132-134;
[10] Jegdi B., Drai D. M., Popi J. P.,Structural Effects of Metallic Chro-
mium on its Electrochemical Behavior,J. Serb. Chem. Soc. 72 (6) (2007) pp.
563-578;
[11] Jegdi B., Drai D. M., Popi J. P.,Influence of Chloride Ions on the OpenCircuit Potentials of Chromium in
Deaerated Sulfuric Acid Solutions, J.
Serb. Chem. Soc. 71 (11) (2006) pp.
1187-1194;
[12] Jegdi B., Drai D. M., Popi J. P.,Open Circuit Potentials of Metallic
Chromium And Austenitic 304
Stainless Steel in Aqueous Sulphuric
Acid Solution and the Influence of
Chloride Ions on them, Corr. Sci. 50
(2008) pp. 1235-1244.
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INSTITUT ZA RUDARSTVO I METALURGIJU BOR ISSN: 2334-8836 (tampano izdanje)UDK: 622 ISSN: 2406-1395 (Online)
UDK: 542:66.087:546.76(045)=163.41 DOI:10.5937/MMEB1502149J
Bore Jegdi*, Maja Stevanovi, Aleksandar Jegdi*
HEMIJSKO I ELEKTROHEMIJSKO RASTVARANJE HROMA NA
SOBNOJ I NA POVIENIM TEMPERATURAMA**
I zvod
Ispitivan je uticaj temperature na elektrohemijsko i hemijsko rastvaranje hroma u kiselim rastvorima
sulfata. Reakcije izdvajanja vodonika, anodnog rastvaranja i hemijskog rastvaranja hroma sa aktivi-rane povrine hroma u rastvoru sumporne kiseline pH 1 slede Arenijusovu zavisnost sa prividnom ener-gijom aktivacije 35 kJ mol-1, 58 kJ mol-1i 62 kJ mol-1, respektivno. Velika energija aktivacije hemijskog
rastvaranja hroma dovodi na povienim temperaturama do znatno izraenije hemijske korozije u odnosuna elektrohemijsku koroziju.
Kljune rei: hemijska korozija hroma, elektrohemijsko rastvaranje, prividna energija aktivacije
*NU Institut za hemiju tehnologiju i metalurgiju,IHTM, Univerzitet u Beogradu, Njegoeva 12,Beograd, Srbija, e-mail: [email protected]
**Ovaj rad je finansiran od strane Ministarstva prosvete, nauke i tehnolokog razvoja Republike Srbije,
Projekat No. 34028.
UVOD
Paralelno sa elektrohemijskim rastvara-
njem hroma odvija i njegovo hemijsko
rastvaranje, koje ne zavisi od potencijala i
koje je u nekim uslovima dominantan proces
rastvaranja, naroito na povienim tempera-turama [1-9]. Hemijsko rastvaranje hroma je
uzrok pojavi izdvajanja vodonika, koje ne
podlee zakonitostima elektrohemijske kine-tike.
U ovom radu vreno je odreivanjeenergije aktivacije procesa hemijskog rastva-
ranja hroma, elektrohemijskog rastvaranjahroma, katodnog izdvajanja vodonika i
temperaturne zavisnosti brzine elektrohe-
mijske korozije. Takoe, odreena je teks-ture hroma primenom OIM metode (orienta-
tion imaging microscopy). Odreivanje
koncentracije hroma u rastvoru je vrenoprimenom atomske apsorpcione spektro-
skopije (AAS). Vreno je snimanje anodnihi katodnih polarizacionih krivih i odrei-
vanje brzine elektrohemijske korozije prime-
nom razliitih elektrohemijskih metoda.
EKSPERIMENTALNI DEO
Pre ispitivanja, povrina uzoraka hromaje mehaniki polirana stupnjevito brusnimpapirom do finoe 1000. Rastvor za ispiti-vanje je bio 0,1 M Na2SO4+ H2SO4, pH 1,
koji je deaerisan provoenjem preienogazota. Za izvoenje elektrohemijskih ispiti-vanja koriena je trodelna staklena elektro-hemijska elija sa vodenim platom zatermostatiranje, sa Pt pomonom elektrodomi zasienom kalomelovom elektrodom(ZKE) kao referentnom elektrodom.
Pre merenja, elektroda hroma je aktivi-
rana katodnom polarizcijom na 0,900 Vtokom 120 s radi uklanjanja povrinskogoksida, koji se na hromu uvek spontano
stvara u dodiru sa vazduhom. Elektro-
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hemijska merenja su izvedena primenom
potenciostata-galvanostata PAR 273.
REZULTATI I DISKUSIJA
Na slici 1 je prikazana inverzna polarna
slika uzorka hroma dobijena OIM metodom.
Plavom bojom je oznaena orijentacija zrna(111) i vidi se da veliki broj zrna na povrinielektrode ima upravo tu orijentaciju. Takoeodreena je veliina kristalnih zrna,koja su
bila malih dimenzija sa velikim stepenom
razorijentisanosti. Zapravo, to su zasebna
zrna sa sopstvenom orijentacijom.
Sl. 1.Inverzna polarna slika, tekstura (111) elektrode hroma dobijena OIM metodom.Orijentaciona veliina zrna se moe proceniti na osnovu razmere date na slici
Ispitivan je uticaj temperature na hemij-
sko i elektrohemijsko rastvaranje elektrode
Cr u temperaturnom intervalu od 291 do
353 K u vodenom rastvoru sumporne kise-
line, pH 1.
Katodne polarizacione krive snimljenena razliitim temperaturama iz oblasti od291 do 343 K u sumpornoj kiselini pH 1 su
prikazane na slici 2. Sa slike se vidi da se sa
poveanjem temperature poveava brzina
izdvajanja vodonika, pri emu je varijacijaTafelovih nagiba sa temperaturom bila u
teoretski oekivanim okvirima. Iseak naslici 2 prikazuje zavisnost logaritma gustine
struje kao funkcije reciprone temperature,
za odreivanje prividne energije aktivacije,za reakciju izdvajanja vodonika na
-0,860 V. Prividna energija aktivacije za
reakciju katodnog izdvajanja vodonika je
35 kJ mol-1
.
Sl. 2.Katodne polarizacione krive za elektrodu hroma u vodenom rastvoru
0,1 M Na2SO4+ H2SO4, pH 1 na razliitim temperaturama
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Na slici 3 su prikazane krive anodne
polarizacije hroma u rastvoru natrijum-
sulfata i sumporne kiseline pH 1 snimljene
na razliitim temperaturama. Oblik polariza-cionih krivih je slian izuzev to su krive naveim temperaturama pomerene premaveim gustinama struje, kao to se moglo ioekivati. Arenijusova zavisnost za -0,750 V
prikazana na iseku na slici 3 daje prividnuenergiju aktivacije za reakciju aktivnog
anodnog rastvaranja od Ea,an= 58 kJ mol-1.
To ukazuje da anodna reakcija vie zavisi odtemperature nego katodna reakcija, totakoe objanjava promenu korozionog
potencijala sa temperaturom u smeru nega-
tivnijih potencijala.
Sl. 3.Anodne polarizacione krive za elektrodu hroma u rastvorusumporne kiseline pH 1 na razliitim temperaturama
Gustina struje elektrohemijske korozije
jkor,elje odreivana ekstrapolacijom Tafelo-vih nagiba na korozioni potencijal i Stern-
Gerievom metodom linearne polarizacije,
dok je ukupna brzina rastvaranja odreivanaanalizom rastvora primenom atomske apsor-
pcione spektrofotometrije juk,an, i prikazane
su na slici 4. Sa slike 4 se moe videti da jeukupna brzina rastvaranja, odnosno ekviva-
lentna gustina struje za sve temperature veanego gustina struje elektrohemijskog rastva-
ranja tj. elektrohemijske korozije. To poka-
zuje da postoji pored elektrohemijskog
znaajno hemijsko rastvaranje hroma nasvim temperaturama. Ta razlika se poveavasa poveanjem temperature kao to se moevideti sa slike 4. Na 353 K ukupna brzinarastvaranja je priblino etiri puta vea negoelektrohemijska brzina korozije. U isto vre-
me to znai da je brzina hemijskog rastvara-nja na toj temperaturi priblino tri puta veanego brzina elektrohemijskog rastvaranja.
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Sl. 4.Ukupna gustina struje dobijena analitiki () i gustina korozione struje dobijena
elektrohemijski na razliitim temperaturama () za elektrodu hroma
Kao to je prikazano na slici 4, ukupnebrzine korozije juk na svim temperaturama
koje su koriene pri ovim eksperimentimasu vee nego elektrohemijske brzine koro-zije jkor,el. Ta razlika je posledica hemijskog
rastvaranja hroma, koji se odvija direktnom
rekcijom molekula vode i atoma hroma sa
povrine elektrode i koji ne zavisi odpotencijala. Razlike izmeu ukupne brzinerastvaranja odreene analitiki juk,anal i
elektrohemijskih brzina korozije jkor,el sunacrtane u zavisnosti od reciprone vred-nosti temperature na iseku na slici 4. Iznagiba te pravolinijske zavisnosti se moeodrediti prividna energija aktivacije i ona
iznosi 62 kJ mol-1
. Visoka vrednost energije
aktivacije ukazuje da je uticaj temperature
na reakciju hemijskog rastvaranja prilinovelik i da je doprinos ukupnoj brzini koro-
zije vei u oblasti povienih nego niih
temperatura. To treba imati u vidu kada se
koriste podaci o brzini korozije dobijeni na
sobnoj temperaturi za predvianje korozio-nog ponaanja na povienim temperaturama.Vrlo je mogue da sline razlike u pogleduuticaja temperature postoje u sluaju koro-zije ostalih metala koji korodiraju i elektro-
hemijski i hemijski (tj. Fe, Ni, Mn i drugi).Hrom se nalazi u pasivnom stanju posle
uranjanja u rastvor za ispitivanje, sa karakte-
ristinim korozionim potencijalomEkor,1kojiodgovara pasivnom stanju hroma. Tek
katodnom polarizacijom na -0,900 V u toku
nekoliko desetina sekundi, pasivni films se
rastvara, pri emu se elektroda aktivira. Tadase formira korozioni potencijal Ekor,2 koji
odgovara aktivnom hromu.
Katodne krive snimljene na pasiviranom
hromu, prikazane u obliku Tafelovog dija-
grama za razliite temperature predstavljene
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su na slici 5. Uoava se da je katodni Tafe-
lov nagib priblino jednak -0,060 V dek-1 ida se gustina struje poveava sa tempe-raturom. Iseak na slici 5 predstavlja Areni-
jusov dijagram dobijen na -0,520 V sa koga
se moe odrediti prividna energija aktivacijekoja iznosi 29 kJ mol-1. To je neto manjavrednost nego energija aktivacije za izdva-
janje vodonika na aktiviranoj povrini hroma(35 kJ mol
-1)
Sl. 5.Katodne Tafelove zavisnosti za pasiviranu elektrodu B sa slike 4.29. na razliitim temperaturama
Prema tome korozija aktivnog metalnog
hroma u vodenom rastvoru sumporne kise-
line pH 1 se sastoji od dva simultana koro-
ziona procesa, elektrohemijske korozije ihemijskog rastvaranja. Pokazano je da se na
Cr elektrodi u rastvoru 0,1 M Na2SO4 +
H2SO4, pH 1 sa povienjem temperatureznaajno ubrzavaju reakcije anodnograstvaranja hroma, hemijskog rastvaranjahroma, katodnog izdvajanja vodonika na
istoj metalnoj povrini, kao i na povrinihroma prevuenoj oksidom.
Reakcija izdvajanja vodonika, reakcijaanodnog rastvaranja i reakcija hemijskog
rastvaranja hroma sa aktivirane povrine urastvoru sumporne kiseline pH 1 u tempera-
turskom intervalu od 293 do 353 K slede
Arenijusovu zavisnost sa prividnom energi-
jom aktivacije 35 kJ mol-1
, 58 kJ mol-1
i
62 kJ mol-1, respektivno. Prividna energija
akti-vacije reakcije izdvajanja vodonika na
hromu prekrivenom oksidom ima vrednost
29 kJ mol-1
.
ZAKLJUAK
Korozija aktivnog metalnog hroma uvodenom rastvoru sumporne kiseline pH 1
se odvija paralelno elektrohemijskim i he-
mijskim mehanizmom. Brzina hemijske ko-
rozije se dobija iz razlika ukupne i elektro-
hemijske brzine korozije.
Reakcija izdvajanja vodonika, reakcija
anodnog rastvaranja i reakcija hemijskograstvaranja Cr sa aktivirane povrine u rast-voru sumporne kiseline pH 1 u temperatur-
nom intervalu od 291 do 353 K slede Areni-
jusovu zavisnost sa prividnom energijom
aktivacije 35 kJ mol-1
, 58 kJ mol-1 i 62 kJ
mol-1
, respektivno. Velika energija aktivacije
hemijskog rastvaranja hroma dovodi na
povienim temperaturama do znatno izra-
7/24/2019 Hemijsko i Elektrohemijsko Rastvaranje Hroma Na Sobnoj i Na Povienim Temperaturama
12/12
Broj 2, 2015. Mining & Metallurgy Engineering Bor
160
enije hemijske korozije u odnosu naelektrohemijsku koroziju. Prividna energijaaktivacije reakcije izdvajanja vodonika na
hromu prekrivenom oksidnim filmom ima
vrednost 29 kJ mol-1
.
Korozioni potencijal je posledica simul-
tanog odvijanja reakcije izdvajanja vodo-
nika bilo na oksidom prekrivenoj povrinihroma sa reakcijom anodnog rastvaranja
hroma kroz pasivni film obrazujui stabilnipotencijal Ekor.1 ili katodnim izdvajanjem
vodonika i anodnim rastvaranjem ogoljene
povrine kada se povrina hroma depasivira
na neki nain (katodnom aktivacijom,mehanikim delovanjem itd.). U tom slu-aju uspostavlja se stabilni korozioni poten-cijalEkor.2.
LITERATURA
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dic discharge of hydrogen on activeand passive chromium surfaces in di-
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[7] Popi J.P., Drai D.M., Electroche-mistry of active chromium: Part II.
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[9] Sukhotin A. M., Khoreva N. K.,
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[10] Jegdi B., Drai D. M., Popi J. P.,Structural effects of metallic chromium
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